1. Technical Field
The present disclosure generally relates to shock absorbers for damping vibration transmitted to a vehicle according to a road state and, more particularly, to a shock absorber capable of changing a damping force according to a displacement of a piston rod.
2. Description of the Related Art
Generally, a vehicle is provided with a suspension system for enhancing driving comfort by buffering impact or vibration transmitted to an axle from a road during driving. One component constituting the suspension system is a shock absorber. The shock absorber is disposed between the axle and a vehicle body. The shock absorber includes a cylinder and a piston rod reciprocating within the cylinder. The cylinder is filled with an operating fluid, such as gas or oil, such that the operating fluid is moved by a piston valve secured to one end of the piston rod to generate a damping force.
A conventional shock absorber has a restriction in that it exhibits constant damping force characteristics with respect to variation of a road state or a driving posture of the vehicle. Therefore, a low damping force characteristic can improve driving comfort but does not ensure stability of the vehicle, whereas a high damping force characteristic can maintain the stable posture of the vehicle but entails deterioration of driving comfort. As such, the conventional shock absorber is incapable of controlling damping force characteristics in response to variation of the road state or the posture of the vehicle.
Accordingly, in order to solve the problem of such a conventional shock absorber, a shock absorber capable of providing variable damping force characteristics according to a displacement of the piston rod has been developed.
FIG. 1 is a cross-sectional view of a portion of a conventional shock absorber.
Referring to FIG. 1, the shock absorber 10 includes a piston rod 14 connected to a vehicle body, and a cylinder 12 secured to an axle connected to wheels. The piston rod 14 reciprocates within the cylinder 12.
The piston rod 14 includes a piston valve 16 disposed at a lower end of the piston rod 14 to divide the interior of the cylinder 12 into a tensile chamber RC and a compression chamber CC. The piston valve 16 is formed with tensile orifices 16a and compression orifices 16b through which the tensile chamber RC and the compression chamber CC communicate with each other. The shock absorber 10 further includes disc valves 18a and 18b disposed on upper and lower sides of the tensile orifices 16a and the compression orifices 16b to elastically deform and generate a damping force according to movement of an operating fluid.
The piston rod 14 has a hollow chamber 20 formed therein. The hollow chamber 20 is provided with a floating piston 22 that can move up and down and divides the hollow chamber 20 into an upper chamber 20a and a lower chamber 20b. The hollow chamber 20 has a first orifice that is defined by a through-hole 24 through which an upper portion of the upper chamber 20a communicates with the tensile chamber RC, and a second orifice that is defined by a shaft hole 26 through which a lower portion of the lower chamber 20b communicates with the compression chamber CC.
As the piston rod 14 is slightly displaced, the floating piston 22 is lifted or lowered, and the operating fluid flows into or from the first or second orifice through the through-hole 24 or the shaft hole 26. As a result, the damping force of the shock absorber 10 is lowered.
The conventional shock absorber 10 reduces the damping force at a low displacement and low speed. However, the hollow chamber 20 has a restricted size so that a range of motion of the floating piston 22 is limited, thereby providing a low effect in reduction of the damping force. Further, when increasing the size of the hollow chamber 20 to improve the effect of reducing the damping force, there are problems of a manufacturing cost increase and a durability deterioration of the piston rod 14. Additionally, the conventional shock absorber undergoes friction between the outer surface of the floating piston and the inner surface of the hollow chamber while the floating piston moves up and down within the hollow chamber to cause an inefficient initial movement and a stick slip phenomenon between the floating piston and the hollow chamber, thereby lowering driving comfort. Furthermore, when the floating piston contacts the upper end or the lower end of the upper chamber or the lower chamber, the conventional shock absorber generates noise, thereby deteriorating quality satisfaction.